Heretofore, various types of washing load size detecting devices for washing machines have been proposed but have presented many difficult problems.
For example, in a washing machine of the pulsator (agitator vane wheel) type in which the fabric articles, particulary clothes, are agitated, there is a type making use of the fact that the rise time required for the pulsator to reach a predetermined rpm varies with the washing load size.
Generally, it has already been known that in the condition in which the clothes are agitated in the washing tub by the pulsator, the load on the pulsator varies to a large extent with various conditions such as the material and shape of the clothes, the degree of entanglement of the clothes put in the washing machine, and the water absorbency of the clothes. Therefore, before the load on the pulsator can be related to the washing load size, agitation must be effected until the aforesaid conditions reach equilibrium, it being necessary to take measurements during the agitation.
On the other hand, the rise time required for the pulsator to reach a predetermined rpm is so short that a single measurement of the rise time of the pulsator is not sufficient to detect the washing load size; thus, a plurality of measurements are required. Therefore, the number of stops of pulsator rotation increases, a fact which is liable to cause damage to the fabrics of the articles being washed depending upon the washing bath ratio. Further, because the measurement is allowed only for a short time, as described above, the accuracy of detection of the washing load size is very low.
There is another type making use of the fact that the average values of the pulsator rpm, the rpm of the motor driving the pulsator, and the current through said motor each correspond to the washing load size; when the washing load size increases, the average rpm decreases and the average current increases.
It is a matter of course that the rpm or current of the motor varies with the load, but it also varies with other factors than the load, such as the motor supply voltage, motor core or winding temperature, and differences in the characteristics of individual motors; thus a judgement for distingishing between a motor load variation and a variation due to external causes, a correction therfore, or stabilization of the voltage is required, so that the arrangement becmes complicated.
On the other hand, the absolute value of the variation due to the motor load is very small as compared with the absolute value of the variation under no motor load, and the accuracy of detection of the washing load size is not good.
FIG. 1 shows the relation between the washing load size and the average of motor current. The character A designates the curve where the motor temperature is 25.degree. C. and B the curve where the motor temperature is 70.degree. C. As is clear from FIG. 1, it is difficult to detect an increase in the motor current due to an increase in the washing load size without the detection being influenced by the temperature.